1. Field of the Invention
The present invention relates to a light source control circuit, an image forming apparatus, and a light source control method, and in particular, to an optical writing control device capable of supporting different types of light sources.
2. Description of the Related Art
Recently, there has been a tendency for promoting conversion of information into an electronic form, and accordingly, an image processing apparatus, such as a printer and a facsimile machine used for outputting electronic information and a scanner used for converting a document into an electronic form, have become key devices. An image processing apparatus having an image capturing function, an image forming function, a communication function, and the like, is often used to form a multi function peripheral (MFP) which is usable as a printer, a facsimile, a scanner, and a copying machine.
Of these image processing apparatuses, electrophotographic image forming apparatuses are widely employed as image forming apparatuses used for outputting electronic documents. An electrophotographic image forming apparatus forms an electrostatic latent image by exposing a photosensitive element to light, and develops the electrostatic latent image to form a toner image using a developer such as toner, transfers the toner image onto a sheet, and discharges the sheet.
In the electrophotographic image forming apparatus, an optical writing device that exposes the photosensitive element to light may employ a laser diode (LD) light source or a light emitting diode (LED) light source. The LD light source includes a light source that emits a beam for exposing the photosensitive element, and a deflector, such as a polygon mirror, for deflecting the emitted beam to scan the entire surface of the photosensitive element in a main scanning direction. On the other hand, the LED light source includes an LED head in which chips of LED light sources are arranged across the surface of the photosensitive element in the main scanning direction.
For an optical writing device, however, designing a circuit of an optical writing control device needs a large cost when the device controls a light source based on information of an image to be formed and outputted. Because there is a difference in the light source that is to be controlled between the optical writing device with the LD light source and the optical writing device with the LED light source, it is necessary, in principle, to design an optical writing control device for each of the optical writing devices. However, because a large cost is required to design the circuit of the optical writing control device as described above, there is demand for an optical writing control device capable of supporting both the LD light source and the LED light source.
In the case of the LD light source, when the trajectory of the beam that has been caused to scan the photosensitive element by the polygon mirror has a deviation from the main scanning direction of the photosensitive element, it is necessary to perform a skew correction process that corrects the skew of an image to be formed. As for the LED light source, when the LED head is skewed with respect to the photosensitive element, a skew correction process to correct the skew and an undulation correction process to correct an assembly error (hereinafter, referred to as “undulation”) of the LED chips assembled in the LED head need to be performed, respectively (for example, Japanese Patent Application Laid-open No. 2008-55717).
Generally, in the skew correction process described above, a skew is corrected by dividing pixels in the main scanning direction into a plurality of blocks and shifting, in units of blocks, a line memory, from which pixel data are read in the sub-scanning direction when the pixel data stored in the line memory are read out and input to the light source device. On the other hand, when the LED light source is used, undulation correction is performed by dividing pixels in the main scanning direction into a plurality of blocks, corresponding to each of the LED chips described above, and then shifting the line memory, from which pixel data are read, in the sub-scanning direction in each of the blocks according to an assembly error correction value of the LED chips when the pixel data stored in the line memory are read and input to the light source device.
In the case of the LD light source, the shift amount of a pixel in the sub-scanning direction in the skew correction process is determined in units of a line. In the case of the LED light source, because it is not necessary to scan the beam in the main scanning direction, it is easy to increase a light emission frequency in the sub-scanning direction, that is, it is easy to increase the resolution in the sub-scanning direction. In addition, the shift amount of the pixel in the sub-scanning direction in the skew correction process and the undulation correction process can be reduced to one pixel or less such as a half of a pixel or a quarter of a pixel.
Meanwhile, some of the optical writing devices include a pattern generating unit that generates various patterns such as a MUSIC pattern for color correction, a process control pattern for process control, and a forgery prevention pattern for forgery prevention. As described above, when the skew correction process or the undulation correction process is performed after the pattern generation, if an image is shifted by one line as is performed with the LD light source, a pattern at the boundary between the blocks collapses, causing a trouble in a subsequent pattern reading process.
In this regard, when the LD light source is used, it is desirable to perform the pattern generating process after the skew correction process is performed. In this case, the generated pattern is not subjected to the skew correction process; however, because local pixels are not shifted even when an image is skewed, the pattern remains to be recognizable over the whole image.
In the LED light source, the undulation occurs due to the assembly error of the LED chips in the LED head as described above. Thus, when a pattern is generated after the undulation correction is performed, because the generated pattern has not been subjected to the undulation correction, the pattern locally collapses due to the assembly error of the LED chips. Therefore, it is desirable to perform the undulation correction after the pattern generation when the LED light source is used.
As described above, the difference in the light source between the LD light source and the LED light source causes a difference in the position to arrange a circuit for implementing a correction process (hereinafter, referred to as a “correcting circuit”) relative to a circuit for implementing pattern generation (hereinafter, referred to as a “pattern generating circuit”). That is, when the LD light source is used, it is desirable to provide the correcting circuit ahead of the pattern generating circuit, and when the LED light source is used, it is desirable to provide the correcting circuit following the pattern generating circuit.
That is, in order to construct the optical writing control device supporting both the LD light source and the LED light source, it is necessary to provide the correcting circuit for the LD light source ahead of the pattern generating circuit and to provide the correcting circuit for the LED light source behind the pattern generating circuit. Consequently, a line memory is provided in each of the correcting circuits. However, when the LD light source is connected, the correcting circuit for the LED light source is not used, and when the LED light source is connected, the correcting circuit for the LD light source is not used.
The line memory is necessary in operating the respective correcting circuits. However, the capacity of the line memory considerably affects the cost for manufacturing the optical writing control device. Thus, it is very inefficient in terms of the manufacturing cost to provide the line memory for each of the cases with the LD light source and the LED light source separately despite that the skew correction process is common to both cases.